Direct-positive photographic materials comprising a supporting substrate bearing a light-sensitive, nonprefogged silver halide photographic emulsion are well known in the art. The Hayashi et al European Patent Application No. 331,185 discloses a silver halide photographic element for forming direct-positive images. The photographic element is particularly adapted for black-and-white computer output microfilm (COM) use. The Heki et al U.S. Pat. No. 4,880,729 also discloses direct-positive image-forming photographic elements. The Heki et al elements are suitable for use in color photographic systems.
It is also well known to employ nucleating agents in combination with silver halide emulsions. For example, heterocyclic quaternary ammonium compounds have been used as nucleus-forming agents in internal latent image type direct-positive silver halide emulsions, and have been used for increasing sensitivity and/or gradation in negative-type surface latent image forming silver halide emulsions. European Patent Applications Nos. 276,842 and 249,239 specifically disclose the use of heterocyclic quaternary ammonium compounds as nucleating agents. Similar nucleating agents are also disclosed in European Patent Applications Nos. 255,009, 278,986, 283,040, 283,041, 311,110, 315,890 and 331,185.
Commonly, the nucleating agents are incorporated in the photographic emulsion by dissolving the nucleating agent in a suitable solvent and mixing the dissolved agent with gelatin and silver halide and any other required components. As an alternative to incorporation in the silver halide emulsion, the nucleating agent can be incorporated in any other suitable layer of the photographic material, such as an interlayer, subbing layer or overcoat layer. However, a significant disadvantage of incorporating nucleating agents in photographic materials by dissolving them in solvents is that the resulting photographic materials exhibit relatively poor incubation stability, i.e., they have a relatively poor shelf life or storage stability.
Accordingly, a need exists for direct-positive photographic materials which contain a nonprefogged silver halide emulsion and a nucleating agent and which exhibit improved shelf life and storage stability.